Replicative fitness in vivo of HIV‐1 variants with multiple drug resistance‐associated mutations

The relative fitness of HIV‐1 viral variants containing a broad range of drug resistance‐associated mutations has been little studied in vivo. Understanding the relative fitness associated with viruses containing mutations may aid future therapeutic management. The aim of this study was to investigate the relative fitness of mutant viruses by assessing a cohort of patients who had developed resistance to many drugs and subsequently stopped all therapy. Eleven patients were assessed for drug resistance associated mutations in the protease (PR) and reverse transcriptase (RT) genes before and, at multiple time points, after stopping therapy. Relative fitness was calculated as a function of the rate of disappearance of mutant viruses when therapy was stopped. The least fit viruses were associated with the RT mutation M184I/V (11.6% less fit) and the PR mutations D30N (12.4% less fit) and M46I/L (21% less fit). Mutations at these codons were associated with significant reductions in fitness levels compared to wild‐type viruses. Mutations at codons 10, 20, 36, and 63 in the PR gene remained fairly constant when therapy was stopped and may not significantly reduce viral fitness. The rapid re‐population of wild‐type viruses may allow the recycling of antiretroviral drugs prescribed previously. J. Med. Virol. 65:218–224, 2001. © 2001 Wiley‐Liss, Inc.